Fix recovery notification - Closes #944

[fchavant]

It appears currentBuild.result can be null if the build is passing.
Ref. https://jenkins.io/doc/book/pipeline/jenkinsfile/#deploy

What was the problem?

It appears currentBuild.result can be null for a successful build.

How did I fix it?

Changed the success condition to check not only for "SUCCESS" but also for null.

How to test it?

• make the build fail (e.g. by using Jenkins' replay feature executing sh 'false')
  • a failure notification should be sent to Slack
• build again normally
  • this time the build should not fail
  • a recovery notification should be sent to Slack

Review checklist

• The PR solves Build recovery slack notifications don’t work #944
• All new code is covered with unit tests
• All new features are covered with e2e tests
• All new code follows best practices

[slaweet]

It works :-) Thank you @fchavant